1. Field of the Invention
The present invention relates to a solid electrolytic capacitor using a conductive polymer as a solid electrolyte layer, and a method for producing the solid electrolytic capacitor. More specifically, the present invention relates to a solid electrolytic capacitor which has a high capacitance per unit volume, and a method for producing the solid electrolytic capacitor.
2. Description of the Related Art
Solid electrolytic capacitors are configured typically in such a way that the surface of an anode body composed of a valve acting metal such as aluminum, tantalum, niobium, titanium, and alloys thereof is roughened by etching to form micropores on the order of microns and thus increase the surface area, a dielectric oxide film (hereinafter, which may be simply referred to as a dielectric film) is formed thereon by a chemical conversion step, a cathode section is further impregnated with a solid electrolyte with a separator composed of an insulating resin interposed between the cathode section and an anode section, a cathode conductive layer composed of a carbon paste and a metal containing conductive paste is formed thereon and then connected to a lead frame to serve as an external electrode terminal, and a resin exterior is formed with the use of an epoxy resin or the like.
Solid electrolytic capacitors using a conductive polymer as a solid electrolyte have been widely used as backup power sources for CPUs for controlling personal computers, because the solid electrolytic capacitors are capable of reducing the equivalent series resistance (ESR) and the leak current, as compared with solid electrolytic capacitors using manganese dioxide or the like as a solid electrolyte. The downsizing of personal computers has been progressed from conventional desktop computers through A4-size notebook computers to B5-size notebook computers. Furthermore, netbooks released into the market last year, which have a limited range of functions to achieve price-reduction and pursue the portability, have been receiving overwhelming support from the market.
Such reduction in size of main bodies of personal computers has been also powerfully promoting the downsizing of circuit board themselves housed in the main bodies, and as a result, there has been demand for products of capacitors which have the same capacitance, a lower profile, and a smaller area, and the demand for improvement of the ratio of capacitance developed per volume has been increased more than ever.
The capacitance of a stack-type solid electrolytic capacitor is proportional to the number of capacitor elements housed in a chip. The stacked capacitor elements are produced by a method of bonding the elements with a silver paste interposed therebetween for the purpose of integration, or a method of piling multiple sheets of cathode sections of elements and covering the cathode sections with a silver paste for the purpose of integration as disclosed in Japanese Patent Application Laid-Open No. 4-91418.
In order to increase the ratio of capacitance developed per volume in such a solid electrolytic capacitor, it is necessary to contain more elements in a specified chip volume, and the reduction in element thickness per element is thus required for stacking more sheets of capacitor elements. However, in view of the thicknesses of an anode body (aluminum foil) and a lead frame which inevitably occupy the space in the chip, the allowable thicknesses for the other components are extremely small, and it is thus becoming more and more difficult to achieve higher capacitances.
As for the thickness of a solid electrolyte layer that is a typical component other than the aluminum foil and the lead frame, a method for reducing the thickness of a solid electrolyte layer in accordance with Japanese Patent Application Laid-Open No. 2006-173593 can be given as an example, in which a step of immersion in a solution containing no oxidizing agent, followed by drying is added to a step of immersion in a solution containing a monomer and a step of immersion in a solution containing an oxidizing agent, followed by drying.
On the other hand, Japanese Patent Application Laid-Open No. 5-304059 discloses a method of joining anode foil pieces (chemically converted aluminum foil pieces) with a conductor strip interposed therebetween for the purpose of integration, then forming a solid electrolyte layer on each anode foil piece, and covering the whole with a conductive paste layer to integrate the cathode section. In addition, Japanese Patent Application Laid-Open No. 3-8313 discloses a method of stacking anode foil pieces with a polyvinyl alcohol thin film interposed therebetween and then forming a solid electrolyte layer in the gap generated by removing the polyvinyl alcohol thin film dissolved in water. In each of the methods in these patent documents, a gap is provided between anode bodies, which will be included in the thickness of a stack of capacitor elements, and a solid electrolyte or a conductive layer is formed in the gap.